Some years ago on a US airline (could have been CO, AA or DL, I don’t remember) I was seated in an exit row and before we left the gate the FA came and asked me if I understood English and was willing to assist in case of emergency. I said yes and nothing happened.
Now would they have moved me if I had not understood English, I don’t know, but I suppose so, because if not, what was the point asking the question?
What is sure is that on AF all orders to the crew (from the Purser or from the Cockpit) are always in French ("PNC aux portes, armement des toboggans et vérifiez la porte opposée"), so I guess this is the language that would be used in case of emergency by the captain, for example if he was to give the order to open the doors. Having somebody at the door understanding what is said in the PA system is certainly an advantage.
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I have noticed that on some other airlines, English is used for this kind of communications, even if English is not the language of the country of the airline. So I suppose English would be used in case of emergency also.
These comments are right on the money. If AF has decided that they will use French in an emergency (and I wouldn't be surprised if they did), then they need people seated in the exit row to speak French as well. If that is the case, then denying you a seat in the exit row was correct procedure.
On many other airlines, the requirement is English, and they have no problem at all if you don't speak the airline's native language. In fact, I was once seated in the exit row on a Lufthansa flight along with a guy who only spoke German. When the FA asked us whether he spoke English and he said no, they booted him out of the exit row and came back with an Irish passenger who didn't speak a word of German, but spoke excellent English (albeit with an interesting accent).